Appendix 14

Discussion

FMDV infection and clinical disease were widespread in the index village and all age groups were affected although the most severe disease, including lameness and the presence of interdigital lesions, was only evident in juvenile cattle. The onset of clinical FMD in Ozbek in September 2004 would appear to be associated with the recent purchase of young cattle from Erzurum market, most probably because of introduction of virus with these animals. However, there was also both epidemiological and serological evidence that an FMD outbreak had occurred in the village in the recent past, approximately 5 months before the present outbreak. Therefore it is also possible that the bought-in calves which were the most severely-affected group in the present outbreak may have developed disease because of being exposed to virus which was already present in the village in persistently-infected or subclinically-infected animals.

For practical reasons neither of the rapid tests evaluated in this pilot study were actually applied “penside” but both were immediately used on return of the investigative team to the LDCC. Both tests were relatively easy to perform although a steady surface and pipetting were required.

The test for detection of FMD viral antigen worked very well when it was used to test vesicular fluid and both specimens were strongly positive for both SP and NSP antigens. However, intact vesicles from which vesicular fluid could be obtained are infrequently observed in field cases of FMD. Furthermore, when “classical” vesicles are recognisably present, the clinical diagnosis is relatively certain and there may be little reason to perform a rapid test. The test performed poorly with OP fluid specimens in that many of the test-strips did not register a positive control line even where the specimen was diluted to reduce its viscosity. Although three specimens gave a very weak positive reaction (“trace” positives), these results had to be considered inconclusive given the absence of a control line on the testing device in each case. In addition it should be remembered that probangsampling is unlikely to be performed during an outbreak investigation and requires the readyavailability of probang devices and considerable experience on the part of the sampler.

The usefulness of a rapid test for FMD antigen, which is only designed to test vesicular fluids and OP fluids and which in fact is only effective in testing the former, must be questioned. To be of use in field investigation it should be possible with a rapid testing method to use epithelial fragments from the edge of lesions as the clinical specimen under test; this would require that a suspension could be prepared from such specimens under field conditions that would be capable of diffusing through the membrane of an immunochromatographic test device. In addition the test devices would have to be sufficiently sensitive to detect the much smaller concentrations of FMD viral antigen that might be expected in such test materials derived from FMDV-infected animals.

Compared with results obtained when the same sera were retested using a laboratory-based ELISA test-kit the rapid test devices for detection of NSP-antibody did not detect very many seropositive cattle. The rapid test was therefore much less sensitive for detection of NSP antibody than the laboratory-based ELISA. However these devices may still be of some use during initial epidemiological investigations at an infected premises or village. In such a situation it is important to estimate the time elapsed since introduction of infection for the purposes of tracing the most likely source of the infection and also to determine the risk of spread associated with different contacts. Establishing which was the first group of animals in a herd or village to become infected can be attempted by estimating the age of lesions in different epidemiological groups within the herd/village and thus identifying the oldest lesion present. This option is no longer available if lesions have healed. However, as a stronger serological response might be expected from animals with healed lesions (due to earlier exposure to the virus) a random-sample could be selected from each group of animals and their serum tested for the presence of NSP antibody. In addition, healing intra-oral lesions which are observed during an outbreak investigation may be caused by trauma or something else other than FMDV infection, as suspected in one of the groups examined during the present study. If the healed lesions were caused by FMD virus some indication of a serological response would be expected whilst no such response would be expected if the injury arose otherwise. Sera from such suspect animals could be collected and tested “penside” for this purpose.

Conclusions

• Rapid tests may be a useful tool during FMD outbreak investigation but they are NOT a substitute for careful clinical and epidemiological investigation • Available tests need further field evaluation and their use “penside” should be attempted.

Acknowledgements

The authors wish to thank staff in the Sap Institute, particularly Beyhan Sareyyupoglu, Oktay Tezal and Yusuf Demir. The authors also wish to thank Princeton Biomedical Corporation for providing test kits.